A ring binder is a stationary accessory, generally called loose-leaf binder in the art. It can protect documents securely and safely, ensure smooth flipping of documents and will not scratch documents. The hook-rings can be tightly engaged. It features an elegant appearance and a stable structure and is unlikely to be scratched or sink in. The whole binder has a high strength and is unlikely to become deformed. A binder generally comprises a long bar-shaped housing, a pair of pivotable hinge boards inside the housing and clamped tightly by the housing, and semi-rings secured on the hinge boards. The semi-rings can be opened or closed by the pivoting of the hinge boards. When they are closed, the ends of the two corresponding semi-rings on the two hinge boards are pressed together, so that the two semi-rings form a complete hook-ring, to facilitate securing and flipping leaflets. When they are opened, the ends of the two corresponding semi-rings are separated, so that the leaflets can be taken out of or added into the binder through the opening between the two ends. The ring binders available on the market currently are opened or closed by using both hands to open the two semi-rings so as to drive the hinge boards to control the pulling force of opening or closing, which requires certain strength of the hands. In addition, the operation by both hands will cause an uncomfortable feel and have the problem of uneven strength between the hands, which is likely to damage the ring binder, resulting in increase in cost. When there are a large number of leaflets within the semi-rings, it will be very inconvenient to open or close the ring binder by both hands.